Typical fuel cell arrangements include multiple fuel cells placed together in a cell stack assembly (CSA). A cathode reactant gas, such as air, and an anode reactant gas, such as hydrogen, are used in an electro-chemical reaction to produce electrical energy. Humidified membranes may separate the anode reactant from the cathode reactant, and conduct ionic current between anode and cathode. A controller monitors operation parameters of the CSA and controls the flow of the anode and cathode reactant gases and the electrical current or voltage to produce a desired CSA power output level.
There are times when the desired power output from the CSA varies. This can be in response to a change in load or power demand. It may also be a result of a change in fuel cell operation such as a transition from startup to normal operation.
CSA durability can be limited by decay mechanisms associated with cyclic operation. For example, voltage cycling may cause performance decay over time. Local membrane humidity cycling may cause the membrane to wear out. Both of these types of cycling may occur in response to changes in load or power demand. While such cycling may result in only modest decay or wearout rates at lower temperatures, the negative effects associated with such cycling is exacerbated by high temperature operation. Therefore, it is desirable to limit the time spent at higher temperatures and the amount of cycling during high temperature excursions. One approach to limiting negative effects from voltage cycling is to use voltage clipping. Voltage cycling may be considered benign below a certain voltage, to which the CSA is clipped. For example, at nominal operating temperatures, it may be acceptable to clip the voltage to a specified value but at higher operating temperatures the voltage clip may not be acceptable. Therefore, voltage clipping is not a complete solution.